1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device in which displays are achromatic.
2. Description of the Related Art
Recently, ST (supertwisted) or SBE (supertwisted birefringence effect) type liquid crystal display devices have been developed, which exhibit a sharp change of the transmitted light in response to the voltage change and a large contrast ratio even when driven with a large number of scanning lines, and have a wide viewing angle. The display device is disclosed in, for example, Japanese Unexamined Publication (Kokai) No. 60-10702, and is of a birefringence-controlled type in which a twist angle of the liquid crystal molecules are made as large as 180.degree. to 270.degree. as compared to TN liquid crystal devices.
FIGS. 1A and 1B are views for explaining a principle of a conventional SBE or ST liquid crystal display device which performs a display based on a birefringence effect. Referring to these figures, the display device includes a liquid crystal cell 5 constituted by first and second substrates 1 and 1' and a liquid crystal composition (not shown) sealed therebetween. On both sides of the cell 5, polarizers 3 and 4 are arranged. Linearly polarized light 103 transmitted through the polarizer 3 is transmitted through the liquid crystal cell 5 and thereby generally becomes elliptically polarized light 101'. At this time, the shape of an ellipse depends on a twist angle .PSI. of the liquid crystal molecules in the cell 5, a retardation value RO defined by an equation .DELTA.n.multidot.d.multidot.cos.sup.2 .theta. (where .DELTA.n is an optical anisotropy of the liquid crystal composition, d is the cell thickness (substrate spacing), and .theta. is the tilt angel of the liquid crystal molecules), and a wavelength .lambda.. The elliptically polarized light transmitted through the liquid crystal cell 5 is transmitted through the second polarizer 4 arranged at a predetermined angle, and is sensed by a human eye. In general, the shape of the ellipse changes in accordance with the wavelength, and the transmittances of the light are different per wavelength. Therefore, the transmitted light becomes chromatic. When a voltage is applied to the liquid crystal cell, alignment of the liquid crystal molecules changes and the optical anisotropy .DELTA.n effectively changes. Thus, the retardation value RO and the transmission of the light change accordingly. A liquid crystal display is performed by using such a principle.
The above-mentioned display devices can be classified into a yellow mode type and a blue mode type, depending on a manner in which the polarizers are arranged. In the yellow mode display device, a bright yellow display is obtained in a non-selected state, and a black display is obtained in a selected state. In the blue mode display device, a deep blue display is obtained in a non-selected state, and white display is obtained in a selected state.
In any of the above conventional liquid crystal display devices, the display is not achromatic. Therefore, a readability evaluation differs in accordance with a visual sense of an observer, i.e., some observers evaluated that readability (e.g., a contrast) was degraded due to the background color. Further, since the display is chromatic, a color display can not be obtained by using a color filter as in a TN liquid crystal devices.
A technique to render the display achromatic is proposed in, for example, JJAP (26, NOV. 11. L177 84 (1987)). This discloses a two-layer-cell liquid crystal device using two liquid crystal cells. The two cells are provided such that the directions of twist of their liquid crystal molecules are reverse, the twist angles of their liquid crystal molecules are the same, and their retardations RO (=.DELTA.n.multidot.d.multidot.cos.sup.2 .theta., where .theta.n is the optical anisotropy of the liquid crystals, d is the substrate spacing, and .theta. is the tilt angle of the liquid crystal molecules) are substantially the same.
This two-layer-cell liquid crystal display device is constructed as shown in FIGS. 2A and 2B. Linearly polarized light 103 transmitted through the polarizer 3 is transmitted through the liquid crystal cell 5 to become an elliptically polarized light 101'. This elliptically polarized light is transmitted through the second cell 6 to become linearly polarized light 102', which is transmitted through the second polarizer 4, and then sensed by a human eye. In this way, the first and second liquid crystal cells 5 and 6 are optically complement each other. Thus, wavelength-dependence of the shape of ellipse transmitted through the first liquid crystal cell 5 becomes complementary with wavelength-dependence of the shape of the ellipse transmitted through the second cell 6. As a result, the light transmitted through the first and second cells 5 and 6 does not have wavelength-dependence, and achromatic display can be obtained. In this case, since the first and second liquid crystal cells must be complement each other, an error between the retardation values of the liquid crystal cells is required to fall within the range of .+-.0.05 .mu.m.
As described above, the two-layer liquid crystal cell liquid crystal display device has advantages in that a b/w display can be obtained and the number of scanning lines can be increased. However, the two-cell liquid crystal device is thick and heavy because two liquid crystal cells are used. Further, the use of the two cells results in very high expenses, when yield is also considered.